Server-Side Template Injection (SSTI) with Grav CMS security sandbox bypass

Summary

Grav CMS is vulnerable to a Server-Side Template Injection (SSTI), which allows any authenticated user (editor permissions are sufficient) to execute arbitrary code on the remote server bypassing the existing security sandbox.

Details

The Grav CMS implements a custom sandbox to protect the powerful Twig methods "registerUndefinedFunctionCallback()" and "registerUndefinedFilterCallback()", in order to avoid SSTI attacks by denying the calling of dangerous PHP functions into the Twig template directives (such as: "exec()", "passthru()", "system()", etc.). The current defenses are based on a blacklist of prohibited functions (PHP, Twig), checked through the "isDangerousFunction()" method called in the file "system/src/Grav/Common/Twig.php":

...
$this->twig = new TwigEnvironment($loader_chain, $params);

$this->twig->registerUndefinedFunctionCallback(function (string $name) use ($config) {
    $allowed = $config->get('system.twig.safe_functions');
    if (is_array($allowed) && in_array($name, $allowed, true) && function_exists($name)) {
        return new TwigFunction($name, $name);
    }
    if ($config->get('system.twig.undefined_functions')) {
        if (function_exists($name)) {
            if (!Utils::isDangerousFunction($name)) {
                user_error("PHP function {$name}() was used as Twig function. This is deprecated in Grav 1.7. Please add it to system configuration: `system.twig.safe_functions`", E_USER_DEPRECATED);

                return new TwigFunction($name, $name);
            }

           /** @var Debugger $debugger */
           $debugger = $this->grav['debugger'];
           $debugger->addException(new RuntimeException("Blocked potentially dangerous PHP function {$name}() being used as Twig function. If you really want to use it, please add it to system configuration: `system.twig.safe_functions`"));
        }

        return new TwigFunction($name, static function () {});
    }

    return false;
});

$this->twig->registerUndefinedFilterCallback(function (string $name) use ($config) {
    $allowed = $config->get('system.twig.safe_filters');
    if (is_array($allowed) && in_array($name, $allowed, true) && function_exists($name)) {
        return new TwigFilter($name, $name);
    }
    if ($config->get('system.twig.undefined_filters')) {
        if (function_exists($name)) {
            if (!Utils::isDangerousFunction($name)) {
                user_error("PHP function {$name}() used as Twig filter. This is deprecated in Grav 1.7. Please add it to system configuration: `system.twig.safe_filters`", E_USER_DEPRECATED);
                return new TwigFilter($name, $name);
            }
...

In the code above it can be seen that the calls of the "isDangerousFunction()" are not performed when the method/filter in the "$name" variable has been considered safe. A function can be defined safe only by an administrator user, by adding it into the configuration properties "system.twig.safe_functions" and/or "system.twig.safe_filters" (a sort of whitelists that by default are empty) of the configuration file "system/config/system.yaml".